In practice any two consecutive rods are usually screwed together in two stages.
In a first stage one is simply rotated relative to the other by a sufficient number of turns for their ends to be correctly engaged with each other.
In a second stage they are locked together by the application of a certain torque between them over a fraction of a turn.
Similarly, they are unscrewed in two stages.
At present different means are usually employed for each of the two operations that are needed.
For the rotation stages these means comprise a chain, for example, of which several turns are passed around one of the rods whilst the other is immobilized. The chain is connected to a capstan so that traction can be applied to it.
The fitting and use of the chain are relatively difficult, time-consuming and dangerous.
Virtually the same can be said of the alternative situation in which a motor-driven chain is merely passed around one rod in a U-shape.
For the locking and unlocking stages a wrench which grips one of the rods between two jaws, the other being immobilized, is usually employed at present.
Screwing and unscrewing machines achieving some degree of mechanization of these two operations have already been proposed.
All have the common drawback of needing to be removed from the workstation when the rods have been screwed together or unscrewed to enable further operations to be carried out on the rods.
Otherwise the lifting device usually employed for these operations is prevented from reaching the rods on which it is to operate.
It is therefore necessary with currently known screwing and unscrewing machines to place the machine around the rods, to carry out the required screwing or unscrewing and then to remove the machine.
Apart from the fact that to enable it to be fitted and removed a screwing and unscrewing machine of this kind must almost inevitably be in two parts adapted to open relative to each other facing the string of rods to be processed, which results in some complexity of construction, these repetitive operations of fitting and removing the machine compromise productivity.